There is currently a large domestic and international market for clean non-polluting generated grid and remote electrical power, such as the electrical power generated by solar energy generating systems. This demand is anticipated only to grow.
Terrestrial solar power systems typically are formed of flat panel photovoltaic (PV) cells, concentrator PV cell systems, or concentrator turbogenerators. Flat panel PV cell systems are advantageous in that they convert either direct or diffuse sunlight to electricity, though direct sunlight is preferred. The panels usually are stationary and the systems can become very large in order to generate sufficient amounts of electricity. Concentrator PV cell systems require fewer PV cells but can only convert direct sunlight to electricity, and therefore require a tracking system and clear skies. Concentrator turbogenerators use concentrated solar energy to heat a medium that is sent through a turbine to generate electricity.
One drawback in the implementation of each of these systems commercially is the expense associated with producing PV cells, tracking systems, and land costs. Moreover, solar power is not economically viable in cloudy regions such as the Northwestern United States or Northern Europe. Though solar power technically is feasible in these regions, the long intervals of low illumination a) drive a requirement for very large energy storage systems to provide power throughout the dark intervals, b) reduce the annual energy output per square meter of solar collection area, and c) do not allow use of concentrator PV cells during the frequent intervals of thin overcast. Thus, in regions of low illumination, solar power is effectively eliminated as a potential clean energy source.
Another market in which solar power is not currently economically feasible is in providing power for military forces, disaster relief, or other mobile applications that require infrastructure. Military forces typically consume large amounts of power, and they often use this power in locations where normal infrastructure either does not exist or is threatened by enemy forces. For example, the Department of Defense (DOD) recently estimated that the actual cost for a gallon of fuel for the US Army in some parts of Iraq is $700 due to the cost of convoy security for tanker trucks or the cost of helicopter airlift of petroleum to remote locations.
Typical terrestrial power systems, such as dams, coal-fired generators, and terrestrial solar arrays, are immobile. These are unsuitable for use by military forces or emergency response agencies. Mobile terrestrial power systems typically rely on fossil fuels, e.g. diesel generators. Though mobile forces often use them, these power systems increase mobile forces' dependence on a steady supply of fuel, which comes at a great expense.
One suggested prior art solution for regions of low illumination is to use very large solar arrays and large energy storage systems. The large arrays produce excess power while the sun shines. This power charges the storage system. When sunlight is not available, the energy storage system is discharged to meet the need for power. Unfortunately, this solution is economically prohibitive as the internal rate of return on the large capital investment is too low for investors. The use of large energy storage adds to the cost of an already expensive system. As a result, this solution is not currently in use.
For the reasons discussed above, most cloudy regions in the world today have no plans to use solar power to meet their energy needs. However, most other energy options fail to meet increasing consumer (or regulatory) demand for environmental stewardship. Nuclear energy remains costly and, in many nations, politically sensitive. Most viable hydroelectric sites are already in use; furthermore, the environmental cost of hydroelectric power is increasingly recognized, resulting in some dams being torn down. Wind energy is economically and politically viable in some areas, but is not sufficiently available in many regions. Fossil fuels like petroleum or coal are becoming more costly and are implicated in global warming; petroleum is also subject to political embargoes or to attacks on oil fields, pipelines, ports, refineries, roads, or tanker ships (cf., the military costs for fuel in Iraq today.)
Referring to FIG. 1, in my aforesaid parent application Ser. No. 12/049,234, I describe an airborne power station comprising an airborne platform or aerostat 50 having a solar power generation system 10 and an electric cable 30 to transport power to a control station 20 on the ground. The airborne platform supports the solar power generation system above the clouds 80 and other atmospheric attenuation. The control station receives the power generated at the airborne power station and distributes the power to, for example, local infrastructure 90.
The airborne platform may be an airship, including a blimp, a semi-rigid airship, or a rigid airship. As shown in FIG. 1, the airship 50 may have aerodynamic stabilizers 55 at the tail. The airborne platform preferably will include controls for the platform's yaw (steering), pitch, and/or roll. Airship embodiments may further include aerodynamic surfaces designed to produce lift when the wind blows.
The solar power generation system may be one or more photovoltaic (PV) cell arrays, optical rectennas, and/or electric generators driven by a solar-heated thermodynamic engine. FIG. 1 shows a PV cell array 10, which may be a flat panel cell PV array or a concentrator cell array, which is supported below the airship by structural elements 40. Preferably structural elements 40 are sufficiently rigid so as to permit pointing of the solar arrays independent of the airborne platform. In alternative embodiments, particularly for use at high altitudes, the solar power general system may be suspended below the airship at a distance, e.g., by cables, where it is less likely to be shaded by the airship. In yet other alternative embodiments, the solar power generation system includes elements that can be steered, e.g. to point more directly toward the sun, in order to maximize the amount of direct sunlight, and consequently, the output of electrical energy. The solar power generation system also includes power conversion equipment that converts power from the form produced by the power generation system to a form better suited for transmission along the power cable. For example, it may convert the low-voltage DC output of a photovoltaic array to high-voltage three-phase power.
The power cables could also function as tethers. Alternatively, one or more tethers may be provided. The system may further include one or more mooring devices to which the power lines/tethers are attached.
Alternatively, as shown in FIG. 2, and as described in my aforesaid U.S. patent application Ser. No. 12/128,561, the power generated by power generation system 10 may be converted by a converter 130 to microwave energy, and beamed to a remote collector 120, e.g. located on the Earth.
Airborne power stations such as described in my aforesaid parent applications provide an alternative to previous energy solutions. Nevertheless, there are challenges related to the implementation of an airborne power station system. For example, the photovoltaic array may not always face the sun at a perpendicular angle reducing power due to cosine loss. Furthermore, the airborne platform may shade at least part of the solar power generation system due to at least some combination of wind direction and solar position in the sky, and the solar power generation systems may shade each other due to at least some combination of wind direction and solar position in the sky.
Referring to FIG. 3(A) these problems may be addressed, in part, by providing a large vertical separation from the airborne platform 700 to a solar power generation system 710 hanging the solar power generation system below the airborne platform, which minimizes the time during which the PV array is shaded. Alternatively, the solar power generation system 720 may be deployed horizontally from the airborne platform 700 on a frame extending from the airborne platform as shown in FIG. 3(B). This latter approach works well when the sun is directly overhead. In yet another solution, the solar power generation system 730 may be mounted on the upper surface of the airborne platform 700 so the airborne platform cannot shade the solar arrays, except from the side, as shown in FIG. 3(C). This latter arrangement often uses conformal (flexible) PV arrays. This latter configuration works well when the sun is high in the sky.
Each of these solutions is not without problems. Large vertical separation from the airborne platform to the solar power generation system does not avoid shading when the sun is directly overhead. To minimize the time of shading, the vertical separation may need to be so large that (a) structural weight of the attachment becomes prohibitive, or (b) extra altitude is needed to lift the solar power generation system above a cloud deck. These problems are worse with larger airborne platforms (as needed for high altitude or high power APS) because larger airborne platforms cast a wider shadow. Solar power generation systems extending horizontally from the airborne platform or from a frame hung below it add mass, and are inefficient when the sun is not near its zenith. The arrays on one side are shaded by the airborne platform when the sun is low on one side, and the arrays shade others on the same side when the sun is low in front or back.
Furthermore, PV arrays mounted on the upper surface of the airborne platform suffer from cosine losses, especially when the sun is low in the sky.